Cancers and tumors are groups of pathologically proliferating cells produced when normal control over cell division is lost. One approach to the understanding of these diseases is to use an appropriate experimental organism to study the control of cell division. The soil nematode Caenorhabditis elegans provides a promising system in which to examine how cell division is regulated in a multicellular organism. C. elegans is small and easily cultured, anatomically simple, and eminently suited for genetics. The post-embryonic development of C. elegans involves a set of rigidly determined and precisely known cell lineages; deviations from the normal program of cell division can be readily detected and exactly defined. Genetic, physical, and chemical techniques will be used to disrupt the normal post-embryonic cell lineages of C. elegans. Mutants with abnormal cell lineages will be isolated. Preliminary experiments have indicated that many types of mutants that undergo abnormal (and, in some cases, lethal) cell proliferations should be obtained; in addition mutants in which certain cell lineages are terminated prematurely will be studied. Anatomical, behavioral, developmental, genetic, and, in a few cases, biochemical characterizations of these mutants should reveal the roles of and interrelationships among those genes which affect cell division. Lesion experiments employing a laser microbeam system will identify intercellular influences on cell division. These experiments should reveal how cell division is controlled in a multicellular organism and the ways in which normal control can be disrupted. I hope that this knowledge will provide a basis for understanding the causes of the cellular proliferation that characterizes cancerous growth.